Signal responsive device



Aug. 9, 1938. H. BELAR 2,125,379

SIGNAL RESPONSIVE DEVICE Filed Sept. 25, 1955 2 Sheets-Sheet 1 INVENTOR HEBBET A-Z/IR BY.

Aug. 9, 1938-. v H. BELAR 2,125,379

SIGNAL RESPONS'IVE DEVICE Filed Sept. 25, 1955 2 Shets-Sheet 2 INVENTOR Patented Aug. 9, 1938 PATENT orrics SIGNAL RESPONSIVE nnvron Herbert Belar, Philadelphia, Pa., assignor to R d o of Delaware Corporation of America, a corporation .Ap piication September 25, 1935, Serial No. 41,987

This invention relates to signal responsive devices suitable forindicating or recording a signal of audio frequency or the like, and has for its principal object the provision of an improved i; apparatus and method of operation for producinga resultant effect dependent on a signal component and a signal envelope component;

It has heretofore been proposed in the photographic recording of sound to reduce the light transmitting area of the record to a minimum by varying the recording beam in response both to the signal and the envelope of the signal. It has been considered necessary to provide in the recording apparatus the usual signal recording channel and, in addition to this channel, a channel including a rectifier, an amplifier and a filter or the like for producing an envelope current which may be supplied either to the signal recording element or to a shutter which independently varies the width of the recording beam. Such apparatus has not been altogether satisfactory, especially from the viewpoint of cost, for the reason that the apparatus is complicated and requires a considerable number of parts. In accordance with the present invenvention, this difficulty is avoided by the provision of means including an amplifier wherein the control circuit is subjected to a potential which biases it substantially to'cut-off when no sound is recorded and during the recording of sound is subjected to a potential which varies in response to the amplitude or volume of the signal to be amplified. As hereinafter explained, this type of amplifier combines the advantages of class A and l class B operation, reduces the number of amplifier tubes required, and makes possible the use of smaller amplifier tubes for the reason that the tube losses at zero signal are greatly diminished.

Further objects of the invention are to provide an improved amplifying apparatus which combines the advantages of class A and class B oper ation, to provide an amplifier wherein the control grid bias potential is responsive to the amplitude of the signal to be amplified, and to reduce the expense and complexity of sound recording apparatus.

The invention will be better understood from the following description when considered in connection with the accompanying drawings, and its scope will be pointed out in the'appended claims.

Referring to the drawings:

Fig. 1 illustrates a sound recorder constructed in accordance with the invention, and.

Fig. 2 is a wiring diagram illustrating certain .1 6 Claims. (01. 179-1003) details of the connections of the apparatus of Fig. 1. l

The apparatus of Fig. 1 includes a sound pickup device or microphone Ill from which signal current is supplied through an amplifier H and leads [2 to operating coil I? of a galvancmeter .forming part of a sound recording head. The

amplifier also supplies through leads M to a galvanometer operating coil l5, a current which varies as the envelope of the signal current. As will be readily understood, a galvanometer recorder or light valve recorder, properly adjusted and supplied with these component currents, will record sound with a minimum of transparent area in the positive record, thus avoiding the background noise due to irregularities in the transparentpart of the record.

As shown in Fig. 2, the amplifier Il may in.- clude single tube stages l6 and l T and a push-pull stage including tubes [8 and 19. Connected in the common plate return lead of the tubes 58-! 9 is a self-bias resistor 23, and a network comprising a resistor 2| and condenser 22 and the plate circuit of a diode triode tube 23. As is well understood, this type of tube combines a rectifier and amplifier tube in one envelope. It will be noted that this tube is coupled to the output circuit of the tube I1 through a transformer winding 24 and has interposed between its cathode and grid a grid bias potential source 25 and a grid leak element including a resistor 26 and condenser 21.

The plate current of tube 23 has its maximum value when no sound is received at the microphone lll. Under these conditions, the potential drop of resistor 2| operates to bias the grid control circuits of push-pull tubes I8 and l 9 substantially to cut-off and the output current of these tubes is maintained at a very low value. When sound is received, however, the tube 23 functions to rectify the audio current delivered to it through the winding 24, this rectified current is transmitted through the resistor 26 and produces a potential in the direction of source 25 thus decreasing the grid potential of the tube and correspondingly varying the potential drop across the resistor 2 i. Since the resistor 2| is connected -in the grid control circuit of push-pull stage l8l$l, the bias potential of this stage is varied in accordance with d the amplitude or volume of the signal. By proper adjustment of these various elements the grid potential of the push-pull stage is caused to vary in accordance with the signal amplitude until the average plate current is approximately half of I sistor 29 and reactor 30 may be omitted. The

output from this amplifier will be undistorted if the variations in bias are limited to the linear range of the tube characteristic and if the bias variation with envelope of signal, is' always great enough to permit the signal modulation to remain substantially within the linear range. the transient state, the rate of change of average bias with time is also important, as it must be slow enough not to cause audible signals in the output lines 12 and at the same time fast enough to prevent audible distortion of suddenly built up louder signals. The timing of the envelope bias potential is determined by resistor 26 and capacitor 21, and also by resistor 2| and capacitor 22. For instance, if capacitor 21 is increased in size, the time required to charge it through the diode portion of tube 23 is increased because this increases the product of RxC,

where R in this case is made up chiefly of the refiected resistance in winding 24, the equivalent resistance of the diode in series with it and the resistor 26 in parallel with the combination. Formulae for the solutions of such circuits being well known, thus the rate at which the bias of tube 23 follows increases in signal strength can be chosen. Moreover, the rate at which the bias returns to the original value after a signal has ceased can be adjusted by varying the value of resistance 26 with respect to capacitor 21. Here, as is also well known, the charge on capacitor 21 will leak off slower if the value of resistance 26 is increased.

The rate of change of envelope potential on the grids of the push-pull stage IB and I9 is further reduced by the action of capacitor 22 in conjunction with. resistor 2| and the plate resistance of tube 23. This additional filtering of the envelope potential is particularly impor tant if the output stage is single ended and not push-pull as shown as well as when the amplifier is used as a recording amplifier supplying also biasing current to a galvanometer, for instance, for noiseless recordings. In this case, the modulation coil of the galvanometer is connected to the output of tubes l8 and I9 through leads I2, whereas current varying in accordance with the envelope of the signal current is derived from the common lead of the anode circuits of push-pull stage [8-49 through leads l4 through reactor 30 to the biasing coil I of the recording galvanometer. Reactor 30 serves to isolate the bias circuit of the galvanometer from the plate circuit at audio frequencies. This is particularly important, if the modulation coil and bias coil of the galvanometer are inductively coupled to each other. In this case, the use of a reactor of sufficiently large value is required in order to prevent the bias coil from becoming effectively a short circuit across the modulation coil. The reactor 30 serves also to further decrease the rate of change of bias current with respect to time.

A recording amplifier of thertype disclosed Inv combines the advantages of both class B and class A output stages. A variable bias output stage requires no more power in the grid circuit than a class A stage but operates on the average at a lower plate dissipation and a higher plate supply efficiency. When used in connection with noiseless recording of sound, it has the additional advantage of avoiding the plate battery drain and the use of output tubes for supplying bias potential. Attention is called to the fact that with the connections described, the galvanometer is always deflected in the same direction and provision for this fact should be made in the galvanometer adjustment.

In ground noise elimination systems as has heretofore been used, it has been customary to set the galvanometer at its mid position when no current was flowing, then to bias it to a minimum position through the output of the anti-ground noise amplifier, and when a signal was applied to reduce this bias in proportion to the signal until at a certain maximum signal, the bias reached zero and the galvanometer operated in its ordinary manner. Since the bias current in the arrangement here described increases instead of decreasing with an increasing signal, it is necessary to adjust the galvanometer with no current input so as to be at the zero position. This adjustment can be accomplished either magnetically or mechanically. In order to adjust the galvanometer so that it at maximum signal will not be biased beyond a mid position about which it can oscillate, it may be desirable to insert a variable resistance 29 in shunt with the galvanometer.

Having thus described my invention, I claim:

1. A sound recording apparatus including a light source, a light control device, means comprising an amplifier for energizing said device, means for biasing said amplifier substantially to the lowest plate current on the linear portion of its characteristic curve, means for varying said bias in response to the amplitude of said sound, and means in the plate supply circuit of said amplifier actuated in accordance with the envelope of signals supplied to said light control device.

2. A sound recording apparatus including a light source, a light control device, means comprising an amplifier for energizing said device, means for biasing said amplifier substantially to the lowest plate current on the linear portion of its characteristic curve, means for decreasing said bias in response to increase in the amplitude of said sound, and means in the plate supply circuit of said amplifier actuated in accordance with the envelope of signals supplied to said light control device.

3. A signal responsive apparatus including a light source, a light control device, means comprising an amplifier for energizing said device, means for biasing said amplifier substantially to the lowest plate current on the linear portion of its characteristic curve, means for varying said bias in response to the amplitude of said signal, and means in the plate supply circuit of said amplifier actuated in accordance with the envelope of signals suppliedto said light control device.

4. A signal responsive apparatus including a pair of amplifiers connected in push-pull relation, means common to the control grid circuits of said amplifiers for producing a potential which varies in response to the signal to be amplified, a pair of leads associated with the output circuit of said push-pull amplifiers for delivering (the amplified signal, and a pair of leads common to the anode circuits of said amplifiers for delivering a current which varies as the envelope of said signal.

5. A signal responsive apparatus including a pair of amplifiers connected in push-pull relation, means including a grid leak element and a diode triode common to the control grid circuits of said amplifiers for producing a potential which varies in response to the signal to be amplified, a. pair of leads associated with the output circuit of said push-pull amplifiers for delivering the amplified signal, a pair of leads common to the anode circuits of said amplifiers for delivering a current which varies as the envelope of said signal, and a galvanometer provided with actuating coils connected respectively to said pairs of leads.

6. A signal responsive apparatus including a pair of amplifiers connected in push-pull relation, means including a diode triode energized in response to the signal to be amplified and common to the control grid circuits of said amplifiers, a pair of leads associated with the output circuit of said push-pull amplifiers for delivering the amplified signal, and a pair of leads common to the anode circuits of said amplifiers for delivering a. current which varies as the envelope of said signal.

HERBERT BEL-AR. 

